{"id":"CVE-2026-44040","aliases":["EUVD-2026-40879","GHSA-j37g-g7hg-vhg6"],"status":"published","title":"UltraVNC vncauth.c uses time-seeded libc rand() to generate VNC authentication challenge bytes","description":"UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce VNC authentication challenge bytes. In rfb/vncauth.c:119-129, the vncRandomBytes() function seeds libc rand() with time(0) + getpid() + rand() and generates a 16-byte challenge. The combined seed space is approximately 31 bits (libc rand() internal state) and is entirely determined by publicly-observable values (wall-clock time and process ID). An attacker who can observe the authentication exchange can enumerate the seed space and predict the challenge within seconds, enabling forgery or offline brute-forcing of responses. Note: on Windows, the active code path may use vncEncryptBytes2.cpp which calls CryptGenRandom; reachability on shipped Windows binaries requires compile-graph verification and is under investigation.","severity":{"version":"3.1","score":4.8,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-10","score":0.00397,"percentile":0.34242,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":false},"cwes":["CWE-338"],"published":"2026-07-01T03:33:20Z","modified":"2026-10-25T17:10:00Z","products":[{"vendor":"uvnc","product":"ultravnc"}],"references":[{"url":"https://uvnc.com/","tags":["vendor-advisory","Product","Release Notes"]},{"url":"https://github.com/ultravnc/UltraVNC","tags":["product","Product"]},{"url":"https://www.securin.io/zero-days/cve-2026-44040-libc-rand-weak-rng-vnc-auth-challenge-ultravnc"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-44040"},{"url":"https://uvnc.com"},{"url":"https://github.com/advisories/GHSA-j37g-g7hg-vhg6"}],"sources":{"cve":[{"id":"CVE-2026-44040","title":"UltraVNC vncauth.c uses time-seeded libc rand() to generate VNC authentication challenge bytes","description":"UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce VNC authentication challenge bytes. In rfb/vncauth.c:119-129, the vncRandomBytes() function seeds libc rand() with time(0) + getpid() + rand() and generates a 16-byte challenge. The combined seed space is approximately 31 bits (libc rand() internal state) and is entirely determined by publicly-observable values (wall-clock time and process ID). An attacker who can observe the authentication exchange can enumerate the seed space and predict the challenge within seconds, enabling forgery or offline brute-forcing of responses. Note: on Windows, the active code path may use vncEncryptBytes2.cpp which calls CryptGenRandom; reachability on shipped Windows binaries requires compile-graph verification and is under investigation.","status":"published","published":"2026-07-01T03:33:20.355Z","modified":"2026-07-09T04:36:05.643Z","cvss":[{"version":"3.1","score":4.8,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium","assessor":"cna"}],"cwes":["CWE-338"],"updated_at":"2026-11-02T13:43:00.288193Z"}],"euvd":[{"id":"EUVD-2026-40879","description":"UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce VNC authentication challenge bytes. In rfb/vncauth.c:119-129, the vncRandomBytes() function seeds libc rand() with time(0) + getpid() + rand() and generates a 16-byte challenge. The combined seed space is approximately 31 bits (libc rand() internal state) and is entirely determined by publicly-observable values (wall-clock time and process ID). An attacker who can observe the authentication exchange can enumerate the seed space and predict the challenge within seconds, enabling forgery or offline brute-forcing of responses. Note: on Windows, the active code path may use vncEncryptBytes2.cpp which calls CryptGenRandom; reachability on shipped Windows binaries requires compile-graph verification and is under investigation.","status":"published","published":"2026-07-01T03:33:20Z","modified":"2026-07-09T04:36:05Z","cvss":[{"version":"3.1","score":4.8,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium"}],"updated_at":"2026-11-02T18:20:04.895477Z"}],"ghsa":[{"id":"GHSA-j37g-g7hg-vhg6","title":"UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce...","description":"UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce VNC authentication challenge bytes. In rfb/vncauth.c:119-129, the vncRandomBytes() function seeds libc rand() with time(0) + getpid() + rand() and generates a 16-byte challenge. The combined seed space is approximately 31 bits (libc rand() internal state) and is entirely determined by publicly-observable values (wall-clock time and process ID). An attacker who can observe the authentication exchange can enumerate the seed space and predict the challenge within seconds, enabling forgery or offline brute-forcing of responses. Note: on Windows, the active code path may use vncEncryptBytes2.cpp which calls CryptGenRandom; reachability on shipped Windows binaries requires compile-graph verification and is under investigation.","status":"published","published":"2026-07-01T06:31:33Z","modified":"2026-07-09T06:33:00Z","cvss":[{"version":"3.1","score":4.8,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium"}],"cwes":["CWE-338"],"updated_at":"2026-11-02T15:09:20.981798Z"}],"nvd":[{"id":"CVE-2026-44040","description":"UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce VNC authentication challenge bytes. In rfb/vncauth.c:119-129, the vncRandomBytes() function seeds libc rand() with time(0) + getpid() + rand() and generates a 16-byte challenge. The combined seed space is approximately 31 bits (libc rand() internal state) and is entirely determined by publicly-observable values (wall-clock time and process ID). An attacker who can observe the authentication exchange can enumerate the seed space and predict the challenge within seconds, enabling forgery or offline brute-forcing of responses. Note: on Windows, the active code path may use vncEncryptBytes2.cpp which calls CryptGenRandom; reachability on shipped Windows binaries requires compile-graph verification and is under investigation.","status":"published","published":"2026-07-01T05:16:20.897Z","modified":"2026-10-25T17:10:00.197Z","cvss":[{"version":"3.1","score":4.8,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium","assessor":"secondary"},{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium","assessor":"primary"}],"cwes":["CWE-338"],"updated_at":"2026-11-02T14:12:27.199837Z"}]}}